How can I prevent a pause at the end of animation loop to have fluid motion.
One common mistake with looping animation is to have the first and last frames share the same values. You want the frame after the last to be the same as the first.

Another cause for pauses in looping animation is to have bezier curves to ease in and out of the first/last frames. Using linear interpolation gives you a constant speed that matches the surrounding frames. Select the keys in the graph editor or dope sheet and press T to change the type.



Two cubes with the same rotation values keyed on the same frames, the red one uses linear interpolation while the blue one uses bezier.